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WHAT'S A WAVE
In each of these science chapters, we'll start with simple ideas and work our way towards the theory. Many of these early sections seem like random topics at first, but they'll eventually fit together.
How does a flag wave? That sounds simple enough, but a waving flag has a certain charm on closer inspection. The pictures below spell out the details in four steps.
The first row shows the side view of a flag as it starts rippling at step one until it's stretched out at step four. The second row looks down on the same flag. (I know; that row looks like a school of sperm, all swimming against the wind, but that's in a later chapter.) A wind never blows at a constant speed, so when it starts:
- A little pressure builds and pushes the cloth to one side. We've labeled the crest of the wave as A. At the same time, the cloth squeezes the air on that side.
- The wind catches the crest of wave A and blows it out towards the end of the flag. Meanwhile, the pressure builds up at the pole on the other side of the flag pushing the cloth in the other direction creating crest B.
- Crests A and B move outwards, and reverse pressure creates crest C.
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